_Rent as a Product Imputable to Land._--The difference between what the labor and capital produce at the margin of cultivation of land and what they can produce on good land, or land that lies within the margin, is clearly attributable to the qualities of the land itself. Given _X_ units of labor and _Y_ units of capital, combine with them no land except such as is too poor to have been previously utilized, and you get a certain product. It is the product of the labor and capital using something which is free to any one. Now put a piece of good land into the combination; to the _X_ units of labor and _Y_ units of capital add a piece of productive land and see what you can create. We do this by taking these units of labor and capital away from the worthless marginal land and setting them to tilling that which is of the better quality. The product is of course larger than they got before, and the difference measures what the land itself adds to the output of the other agents in the combination. The true conception of rent is that of the specific addition which land makes to the product of other agents used in connection with it. There are various ways of measuring this addition, but the method just used will at least show that the presence of the good land is the cause of the excess of product which given amounts of labor and capital secure over what they could create on land of the poorest quality. _Rent as a Differential Product._--In the early statements of the rent law it was not said that the rent of a piece of land is the product specifically attributable to it. If it had been, the chances are large that a much broader and more scientific use of the rent formula would have resulted. The law of rent, as it was actually stated, made it consist of a differential amount. It was what a given amount of labor and capital would produce under one set of conditions minus what they would produce under another. Since it is the presence or the absence of the productive land which makes the only difference between the two conditions, rent, even as it is thus defined, is really the amount of product specifically attributable to the land. It is what is created when the land is used in excess of what would be created if it were not used and if the cooeperating agents did the best they could without it. We may use, as the most general formula for the rent of land, the contribution which land itself makes to the product of social industry.